The present invention relates to a tread for a pneumatic tire. Pneumatic tires are a laminated mechanical device of generally toroidal shape having beads and a tread and made of rubber, chemicals, fabric and steel or other materials. When mounted on the wheel of a motor vehicle, the tire through its tread provides traction and contains the fluid that sustains the vehicle load. In particular, the tread of the present invention is particularly suited for a radial-ply tire. The term "radial-ply" tire is intended to include a belted or circumferentially-restricted pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 65.degree. and 90.degree. with respect to the equatorial plane of the tire. The tread is provided with high density zones located in the shoulder region of the tread.
Heavy duty radial truck tires are often characterized by tread patterns utilizing ribs with projecting and reentrant portions in a generally zig-zag pattern. This tread pattern has proven advantageous for a variety of performance criteria. However, this tread pattern has also proven to be susceptible to a type of irregular wear known as "railway wear" or "river wear". River wear is wear in a circumferentially extending rib of a tire tread along the edge zones. The irregular wear spreads across the tread creating an unacceptable appearance.
In the prior art, tread designers have sought to eliminate or reduce river wear by placing narrow slots in the edges of the tread ribs as in U.S. Pat. No. 4,449,560. Tread designers also modified projecting portions of the tread rib to prevent the initiation of river wear, as in U.S. Pat. Nos. 4,266,592 and 4,200,134. While these techniques have been moderately successful, river wear continues to be a significant source of irregular wear for truck tires used extensively for long distance highway applications.